Mariek, I put to 02 in first because of the pressure in the 02 storage bottles. The max pressure is only about 2500psi in a 350 cu.ft. bottle. So if you put the air in first you have to top off with 02. If you have a 3000psi tank you would have to put in 2583psi of air and then 417psi of 02 to get a 32% mix. That would mean that you have to boost the 02 (something I don't like to do at high pressure). If you go the other way you would be boosting the air, which seems allot safer to me.Also I don't see how putting the air in first makes a difference, you are still passing the pure 02 into the value(to me the value is more a danger to combustion then the tank is). If you guys are really interested in this you should take a blending class from JW.